1. Field of the Invention
This invention relates to gas turbine engines and more particularly to the structure supporting an outer air seal about an array of rotor blades in such an engine.
2. Description of the Prior Art
A gas turbine engine has a fan section, a compression section, a combustion section, and a turbine section. A rotor extends axially through the turbine section. A row of rotor blades extend outwardly from the rotor. A stator circumscribes the rotor. The stator includes an engine case and an outer air seal supported and positioned by the case. The outer air seal is radially spaced from the row of rotor blades leaving a tip clearance therebetween. Working medium gases are pressurized by a fan section, compressed in the compressor section, burned with fuel in the combustion section and expanded in the turbine section. The temperatures of the working medium gases discharging from the combustion section into the turbine often exceed fourteen hundred degrees Celsius (1400.degree. C.).
The hot gases entering the turbine section lose heat to the turbine blades and the case. The turbine blades are in close proximity to the hot gases and respond rapidly to temperature fluctuations of the gases. The outer case is remotely located with respect to the hot gases and responds more slowly to temperature fluctuations than do the rotor blades. The outer air seal is positioned by the case and responds with the case. Accordingly, the tip clearance between the outer air seal and the row of rotor blades varies during transient operating conditions. A substantial initial clearance is provided between the outer air seal and the blade tips to prevent destructive interference. Resultantly, the clearance at equilibrium conditions is larger than desired and a portion of the working medium gases leaks over the tips of the blades. Such leakage of medium over the blade tips limits the obtainable stage efficiency and engine performance.
In modern engines, the tip clearance between the rotor blades and the outer air seal is reduced by cooling a portion of an engine case. A cooling medium, such as air pressurized by an upstream compression stage, is typically used for the cooling. U.S. Pat. No. 4,019,320 to Redinger et al. entitled "External Gas Turbine Engine Cooling For Clearance Control" is representative of structures in which the diameter of an outer air seal is reduced by cooling a portion of the case. As shown in Redinger et al., the engine case has massive external flanges and large internal rings. The large internal rings support the outer air seal. These continuous rings are flanges extending inwardly from the engine case and are support rings rigidly bolted to the inwardly extending flanges. As the cooling medium carries heat away from the external flanges, the external flanges contract and force the internal rings and the outer air seal to a smaller diameter. The tip clearance decreases and increased turbine efficiency results.
Athough increased turbine efficiency results in increased performance, the increase in performance is diminished by the use of cooling air. To pressurize the cooling air, the gas turbine engine uses energy; energy that otherwise might be used for propulsion. Any reduction in cooling air consumption reduces the performance penalty caused by the work of pressurization. A support structure having a fast response time enables the turbine to reach quickly the desired level of turbine efficiency. A faster response time causes a faster decrease in the tip clearance. An improved support structure having a fast response time and requiring smaller amounts of cooling air to obtain a given outer air seal displacement is needed. Such an improved support structure increases the sealing effectiveness of the outer air seal. A more effective outer air seal results in a more efficient machine. The need to produce energy efficient machines has grown in recent years because of increased fuel costs and limited fuel supplies. Accordingly, scientists and engineers are working to design a support structure for use in externally cooled turbine sections that will increase the sealing effectiveness of the outer air seal.